Introduction

As engineers and scientists simplify and ease our lives by discovering, innovating, and designing and creating newer products and better technologies, we often miss how our current technological prowess and abilities are a result of major discoveries and innovation in supporting industries and processes.

A primary example is the smelting industry that extracts the metals that give structure, shape, and form to a diverse range of appliances, tools, and day-to-day items.

A simple revisit to the industrial revolution shows us how the foundational metal (Iron), and its economical mass extraction and production was crucial to bringing us the mechanical age. It would have become impossible to sustain our progress (leaps and bounds) if we had not found a more potent metal to serve the needs of the era for a more economical replacement.

We plowed through to a newer age within a century because of the discovery of a new metal, and the innovation of developing the needed processes for its economical extraction.

This was achieved by the rediscovery of a metal that was plentiful and relatively inexpensive; was light weight yet strong; proved a good conductor of heat and electricity; naturally resistant to corrosion; and was both ductile and malleable allowing for creation of different alloys to cost-effectively match the exact needs of the products it needs to be used with.

Serving all the needs and requirements of our modern age is the metallic element aluminum, the third most plentiful element present in our earth’s crust. Taking up nearly 8% of earth’s soil and rocks, metallic aluminum is always found as a chemical compound. It bonds with other elements such as oxygen, sulphur, and silicon.

This poses a problem: only pure metallic aluminum offers the properties needed to meet the demands of our current age.

Over the past hundred years, since the dawn of the automobile revolution, engineers and scientists have been continuously developing and refining methods for extracting pure metallic aluminum from earth’s crust. The current best and most economic method of extracting pure, metallic aluminum is by extracting it from the aluminum oxide ore.

But before we delve into the working of the processes that are used to extract this fine metal, let’s take a moment to understand why aluminum was “re-discovered”. Knowing this is important on two accounts:

  1. It allows us to realize that the ancient civilizations were well aware of its presence and uses. This raises the question: why did they not place as much importance to it as we are, which allows us to:
  2. Understand the importance of the metal in context of our modern and unprecedented progress in scientific discovery, engineering and technology. Aluminum became important because of the demands of our age — speed, durability, and economic efficiency.

Let’s see some of the discoveries about how the metal was being used in the ancient civilizations.

Aluminum — A Brief Journey Across History

The ancient civilizations realized the importance of Aluminum compounds, with evidence dating back to as early as 5000 B.C. Persia. The Persian potters were renowned for strong vessels made from clay, which it was found contained aluminum oxide — an integral component of the modern aluminum extraction processes.

The ancient Babylonians and Egyptians knew the effects different compounds of aluminum produced. We have found that they extensively used them in their medicines, for dying fabrics, and in cosmetics.

However, none of them actually tried to extract the metal from its oxide until the nineteenth century when aluminum was first identified and extracted as a separate element for the periodic table. The early techniques and methods used for isolating the metal were difficult and costly, making aluminum one of the rarest elements on the planet. Although we have a curious case of a metal ornament found in the tomb of Chou-Chu, a military leader in 3rd century China, and which contained 85% aluminum. How it so much aluminum was extracted and efficiently used remains a mystery, and considered a special case.

The discovery of the fact that aluminum oxide actually contained a metal (till then considered rare) was only made at the end of the 1700s. It defeated all attempts at being extracted.

Many of the greatest minds of the era set out to extract this unusually stubborn metal.

The first documented attempt for isolating and extracting aluminum is from Humphry Davy. He tried to remove potassium and sodium from the “earths” (an early name given to the oxides) using electric current, however his attempt to use the same process to release aluminum from its oxides failed.

In 1825 Hans Christian Oersted from Copenhagen, Denmark, became the first person to make a successful attempt by heating aluminum chloride with potassium. His attempt made headway but only procured an impure sample of aluminum.

Hence, it fell to Friedrich Wöhler, a German chemist to perfect Oersted’s method in 1827. Replacing sodium with potassium and heating the oxide, he was able to obtain pure aluminum for the first time in history.

INTERESTINGLY SO, aluminum became and remained as valuable as silver for over half a century after being discovered. So much so that in 1884, when around 60 Kg (125 lb) of aluminum was produced in the United States, it was sold for the same price of silver. Compare this with the dramatic change in pricing in the year 1995 when 3.6 million metric tons (7.8 billion lb) of aluminum, produced in U.S. plants was sold at a unit price SEVENTY-FIVE times LESSER than the price of silver.

The world had discovered, developed, refined, and nearly perfected an economical extraction method of pure aluminum from its core.

Credit goes to the ideas and independent work done by two 22-year-old scientists in 1886. They developed a smelting process that would make mass extraction of aluminum economical. The processes are known as the Hall-Heroult process. Named after the American and French inventors, it is still the primary method of aluminum production today. This was followed by contributions by the Austrian chemist Carl Josef Bayer in 1888. The Bayer process dramatically increased the refining process for the aluminum ore and further bolstered our ability to economically produce aluminum.

Producing Aluminum — The Two Phases

The extraction and production of pure metallic aluminum can be divided into two distinct phases:

  1. Using the Bayer process of refining the primary aluminum ore (bauxite ore) to obtain the aluminum oxide
  2. Using the Hall-Heroult Process of smelting the aluminum oxide to release the pure metallic aluminum for further processing

Let’s start with the raw materials needed to start the process.

Producing Aluminum — It begins with the Raw Materials

Being the second most abundant metal in the earth’s crust does not make aluminum easy. In general, Aluminum compounds occur in all types of clay, but towing millions of tons of clay is not the option.

Hence, the basic premises of economical extraction and production are the availability of large ore deposits where the metal is available concentrated quantities and less number of impurities in the ore. Fewer impurities ensure that the process can easily cater the impurities that are present and easily remove them as either sludge, or through another refining process.

Bauxite is the most useful ore used for producing aluminum.

It consists of 45%-60 of aluminum oxide per unit. The remainder comprises of various impurities (such as potassium, iron, and other metals) which can easily be removed as sludge that is left at the end of the process. Bauxite is normally available as clay and soft dirt, and although bauxite deposits can harden to rocks. Bauxite is easily available in the upper layers of the earth crust, and hence is mined from a few meters below the ground. Bauxite is usually dug from pit mines created for extracting the ore. Currently, Australia leads in supplying bauxite to the world — commanding and serving over one-third of the world’s appetite for more raw bauxite ore.

Other raw materials include caustic soda (sodium hydroxide) for dissolving aluminum compounds and separating the impurities. Other chemicals and compounds are also used depending on other identified impurities present in the ore.

Producing Aluminum — The basics

It takes about 2 kg (4 lb) of bauxite to deliver 0.5 kg (1 lb) of pure metallic aluminum, or to extend it to more practical and industrial process worthy levels, about four tons of bauxite are needed to deliver 2 tons of alumina, and those 2 tons of alumina are then refined to produce 1 ton of pure aluminum.

Let’s see how this is achieved in two stages:

Stage 1

Converting Bauxite to Alumina

This is a five step process:

#1 — Crushing the Bauxite Ore

The purity of the final material depends on the predictable nature of the ore as much as on the systematic nature of the process.

Hence, the recovery of the alumina starts by filtering the raw bauxite by passing it through various filtering screens. These sort the bauxite ore by its grain size. Different grains are collected in separate containers and then crushed so that all of them have a uniform grain size.

The crushed ore is moved to large grinding mills that further grind it into finer particles. As the ore is being crushed, a caustic soda (sodium hydroxide) is added to create a solution of bauxite and sodium hydroxide. The mixture is allowed to heat under high pressure and temperature.

To keep the temperature consistently spread across the ore, the mill continuously rotates until a consistent liquor-like mixture is achieved for further processing. It is called the “slurry” and contains sodium aluminate, trace elements, and un-dissolved residues from the bauxite ore containing titanium, iron, and silicon. The residues, called “red mud” are allowed to settle at the bottom from where they are easily collected.

#2 — Digesting Slurry

The extracted slurry (without the red mud) is carefully pumped into a digester. The digester is another container where chemical reactions are performed at temperatures of 145 °Celsius (300 °Fahrenheit) and pressures of 50 lb/inch. This dissolves the alumina in the slurry. The conditions are achieved over a period ranging from half-hour to several hours.

During the digesting, additional sodium hydroxide is added for dissolving compounds containing aluminum. As a result, additional compounds either combine with other compounds or dissolve in the sodium hydroxide solution. This creates a layer of scale on the equipment which is cleaned periodically.

The end result of the process is a solution of sodium aluminate. Given that the process occurs inside a pressured tank, the conditions (pressure and temperature) of the process are maintained by transferring the slurry into multiple flash tanks. Once the heat and pressure is stabilized, it is pumped into “settling tanks.”

#3 —Settling the Slurry

Trace elements and compounds formed during the digesting process are allowed to sink towards the bottom of the tank primarily due to gravity, and with aid from additional chemicals. This allows the coffee colored liquor to rise to the top.

The warm alumina contains tiny and suspended crystals and some other particulate impurities. It is directed through a series of giant cloth filters. Known as “leaves”, these filters are used for filtering the remaining and undesired particulate matter present in the liquor.

The material caught in the “leaves” is called a “filter cake”. This material is again washed to recover caustic soda and alumina. The new filtered liquor is cooled before being moved to the “precipitators.”

The remaining red mud is also pumped to a different storage pond after being washed (this allows us to recover alumina and caustic soda). The red mud is dried via evaporation.

#4 Precipitation

The aliminate solution (the liquor) contains crystals that have to be solidified via evaporation. Extreme care is needed to achieve the right results at the end of the process. This is done in rows of HUGE tanks (think six-story high) in which the clear sodium aluminate is pumped and then precipitated into crystals.

The process begins by adding “seed crystals”, fine particles of alumina (alumina hydrate). This causes the pure alumina particles to fully crystallize as the liquor is allowed to cool. Each seed forms the epicenter, allowing the alumina crystals to grow around it and slowly settle to the bottom of the tank.

The crystallized particles are regularly removed from the precipitators and transferred to “thickening tanks.” Once all the crystals are collected, they are filtered and moved to the “calcination kilns” via conveyor belts.

#5 — Calcination of the Crystals

The precipitated alumina hydrate is consistently filtered and washed to further purify it and to remove any impurities. However, the alumina hydrate that crystallized in the precipitation process contains water. It has to be removed.

Calcination is the process of removing the water content from the crystals. The resultant crystals are known as anhydrous alumina.

The hydrated crystals are move to the calcining kiln using a conveyor. The calcination kiln is brick-lined and gas-powered to reach and sustain temperatures of 2,000 °F (1,100 °C) for drying the crystals. Once again, to achieve consistency, the kiln is mounted on a tilted foundation and rotates slowly.

Traditional calcinating kilns have cooling equipment in their foundation and allow the alumina to move to the foundation for cooling purposes. Newer plants have begun using fluid beds where the alumina particles are calcinated while being suspended above a hot air screen.

THE RESULT of calcining is pure alumina, a white, finely grained powder. The alumina is now ready for smelting into aluminum, whereas the caustic soda is recycled and pumped to the beginning of the process.

This brings us to the end of the Bayer process. To recap, here are the steps:

THE BAYER PROCESS (Extended)

  1. Bauxite ore is mechanically crushed and filtered until we have a fine slurry
  2. Slurry is pumped into a pressure cooker like structure, the digester. Aluminum is dissolved using caustic soda and other chemicals to create liquor like solution.
  3. Hot slurry (solution of sodium aluminate) is passed through a series of flash tanks for maintaining heat and pressure
  4. Hot slurry is pumped into a settling tank. Settling leaves red mud at the bottom of the tank.
  5. The liquor is again filtered using giant “leaves”. Filter cake is washed and dried.
  6. Filtered liquid is pumped to precipitators. Seed crystals are added to start precipitation. Crystal precipitate is moved to calcinating kiln to remove water molecules.
  7. Anhydrous alumina (white powder) is achieved at temperature of 2,000° F (1,100° C). Crystals are cooled.

Let’s move to Stage 2: Smelting

Stage 2

Smelting Powdered Alumina to Aluminum

The Hall-Heroult process is the process of smelting (reducing) the anhydrous alumina into pure aluminum. This occurs in a “reduction pot”, a large steel contained completely lined with graphite carbon. Industrial plants often use pots line in long rows to increased efficiency and productivity.

Let’s start with the basic ingredients for the process:

Smelting Alumina — Ingredients

Primarily, there are three ingredients to the Hall-Heroult process, namely:

  1. Cryolite
  2. Carbon
  3. Electricity

Cryolite

Cryolite is a chemical compound made of aluminum, fluorine, and sodium. It is used as an “electrolyte” for the “electric cell” in which the alumina is added. An electrolyte is simply a solution that allows effective flow of current, whereas the whole tank/chamber used for the process acts like a liquid battery used in our cars.

Earlier processes relied on naturally occurring cryolite, however now it is mass produced synthetically for use in the smelting of aluminum. The aluminum fluoride present in the mixture is used for reducing the melting point of the electrolyte solution. Significantly increasing efficiency by reducing resistance to current (and consequent loss of energy as heat).

Carbon and Graphite

The reduction pot in the Hall-Heroult process is an electric cell, and hence requires electrodes .This is where carbon/graphite comes into play. They are used as electrodes that carry current through the electrolyte. Because of the high temperatures reached during the smelting process, the carbon/graphite electrodes react with oxygen to form carbon dioxide.

This causes them to be used during the process. Every 22 kg of aluminum produced consumes as much as 2 kg of carbon. Hence, the carbon rods need to be periodically replaced to keep the process running until all of alumina is transformed into pure aluminum metal.

Electricity

The Hall-Heroult smelting process consumes large amounts of electricity. The process cannot even start unless the reduction pot is connected to an electric source capable of supplying (on average) 15 kilowatt-hours (kWh) of energy of Direct Current. The normal alternating current (used in homes) is of no use in a reduction pot. Ideally, the DC current of 15 kWh can generate up to 1 kg (2lb) of pure metallic aluminum. As a result, electricity can costs can come to represent as much as about one-third of the cost of completing the whole Hall-Heroult process of smelting aluminum.

Let’s put each of these ingredients in their proper context

Smelting Alumina — Setting the Reduction Pot

The key to the chemical reactions occur in the Hall-Heroult process and which transform the alumina into metallic aluminum is running a controlled amount of electrical current through a mixture of alumina and cryolite.

The electric cell (reduction pot) is first filled with a mixture of alumina and cryolite. The carbon and rods are suspended into the mixture before being connected to a Direct Current (DC) source. Using the reduction pots requires tremendous amount of power, a simple reason why the smelting plants are always built near power plants, or areas where an assured and steady supply of electrical power is available.

The reduction pot works by allowing the cryolite and alumina mixture to break down into positive and negative charges which are then collected on the electrodes. The electrode connected to the positive terminal of the electrical circuit is called the Anode, whereas the carbon electrode connected to the negative terminal is called the Cathode. Anode attracts negatively charged particles, and vice versa for Cathode. The carbon anode is normally made of petroleum pitch and coke, whereas the cathode is the thick carbon or graphite lining of the pot. This significantly increases the surface area for the pure aluminum to collect on.

So, the reduction process is the process of transferred charges by allowing electricity to flow between the two electrodes using the cryolite/alumina mixture.

Now, the electrical voltage (the force that pushes the charges) used by a typical reduction cell is about 5.25 volts. However, the amperage (or the number of charges flowing per second) is extremely high. It normally is in the range of 100,000 to 150,000 amperes, and at times even more. This is primarily the reason why the carbon electrode dissolves during the process — conducting so may charges at a time heats them up, causing them to react in the oxygen released during the reduction.

Smelting Alumina — Passing the Current

When the reduction pot is ready, the DC current is allowed to flow through the cell. This cause the Anode (positive) to react with the oxygen in the alumina, forcing the carbon molecules of the anode to form carbon dioxide, leaving a single pure metallic aluminum to settle at the bottom of the pot.

The aluminum that settles at the bottom is regularly siphoned and collected into special crucibles alongside the carbon dioxide gas. The gas escapes, leaving pure aluminum.

Normally, the loss in the original cryolite is negligible, whereas the alumina is constantly replenished from the stage 1. The temperature requirements also play a crucial role in how effectively the process proceeds. Normally, pure metallic aluminum begins to form at 900°C, however, once the metal is formed, its melting point is just 660 °C. This is because the alumina’s oxygen content dramatically increases its melting and boiling points, whereas pure aluminum has a much lower melting point. As a result, it is very important to regularly siphon the collected aluminum before it reaches its melting point as it can once again combine with the sludge left by the carbon electrodes.

Smelting Alumina — Purity Levels

Now, the aluminum produced is normally offers a purity level of 99.7% pure, an acceptable purity level for most applications. However, some smelters use more refined techniques for smelting super pure aluminum (achieving 99.99% purity). This aluminum is in high demand for special applications where high conductivity, malleability, and ductility is needed.  Although the small variation (of only 0.29%) may seem marginal, it can significantly alter the properties of the aluminum produced in the end.

Smelting Continuity — Problems

The Hall-Heroult process’s smelting process has to remain continuous to remain profitable as halting and restarting the process is extremely costly, not to mention affects the integrity of the structure. Hence, the plants remain in production 24 hours every day, running year-round to both remain healthy and competitive.

This introduces another problem: of continuous supply of electricity. If the production is halted due to a power failure or interruption for more than just four hours, the aluminum that has been collected at the bottom of the reduction pot solidifies. Re-melting the metal within the pot is often not possible, requiring expensive rebuilding processes that can cost up to $1.6 billion (a typical cost for a modern smelter).

To keep the smelters economically operational, many smelters redirect the heat that is generated during the process for collecting larger quantities of aluminum per siphoning. Another innovation is the use of recycle material into the process. Given that recycled metal requires 5 per cent of the energy required to make new metal, it allows significant savings in the overall energy costs for the smelter, WITHOUT making any difference to the primary metal.

Once the process has been completed, the collected metal can be forged into various shapes, forged, or extruded into the shapes needed to make various items (ranging from electronics and appliances, to cans, automobiles, airplanes, and much more).

Properties of Aluminum

The metallic aluminum that has been collected using the two stages of extraction and smelting is the second most abundant and used metal on the planet. Let’s not forget why that is the case. Aluminum offers a holistic range of valuable properties:

Aluminum is Strong and Light Weight

A metal is not useful if it is not strong enough to hold the integrity of a structure. Then again, the metal should be able to do this without impeding the mobility of the end product. Aluminum has a density of 2.700 kg/m3 which is one-third of steel, whereas its malleability allows it to easily form alloys that dramatically increase its tensile strength.

This is the reason why it is being so readily chosen as the choice for aircrafts, automobiles, structural designs, and appliances. Aluminum alloys can offer tensile strength from anywhere between 70 to 700 MPa.

Aluminum Experiences Linear expansion

This allows predictability in its reaction to variations in temperatures. Aluminum is known to withstand low temperatures without becoming brittle, making them exceptional choice for use in cold regions, whereas their linear and relatively large coefficients of linear expansion allows the designers and engineers to easily calculate circumstances/conditions where it will affect their design.

Aluminum Offers Exceptional Machining Capabilities

It is easy to work with aluminum using most of the machining methods — including cutting, punching, drilling, bending, and milling, among others at comparatively lower energy costs.

Aluminum has great Formability

Aluminum’s metallic structure makes it malleable (it can easily be drawn into sheets and foils), and ductility (it can easily be drawn into wires). The malleability is essential for extrusion purposes where the metal can be rolled in either condition (either hot or cold). This property is commonly exploited during rolling of foils and strips, and also when bending it or performing other forming operations.

Additionally, aluminum also features easy joining, making it a good choice for profile design where jointing is essential, proving itself a good joiner with bonding, taping, friction stir welding, and fusion welding.

Aluminum Offers high Conductivity

Aluminum is a superb conductor for electricity, a property exploited in its extraction. When coupled with its reduced weight, aluminum makes for a great conductor, offering as much as half the weight of normal conductors.

Aluminum is Not Toxic

Aluminum is the most common element in our earth’s crust after silicon and oxygen. Compounds of aluminum also naturally occur or are present in the food we eat. This makes it a great choice for household appliances or for use in hand-held devices or ones that are used regularly.

Aluminum Promises Resistance to Corrosion

Aluminum naturally combines with environmental oxygen to form a thin layer of oxide on its surface without affecting its integrity or strength. This layer is dense, and hence offers excellent resistance to corrosion. Thanks to natural affinity of the aluminum to oxygen, even if the layer is damaged it is automatically repaired.

Normally, Aluminum is neutral in slightly acid environments and extremely durable. However, environments where acidity or basicity is high, corrosion is rapid. This is overcome by anodizing the upper layer. Anodizing increases the thickness of the layer, significantly enhancing its resistance to corrosion under harsh conditions.

Aluminum is Non-Magnetic

Aluminum is often termed a paramagnetic material. Though it conducts electricity, it does not react to magnetic effects. This makes it a great choice for devices and areas where magnetic fields are present but where integrity of the equipment must be maintained (e.g. in magnet X-ray devices).

The Magic of Aluminum —Transformation into products

Smelting is not the end of the line for aluminum. It is just the beginning. Remember that this aluminum itself is now a raw material for use in the industry. So, once smelted, where does aluminum go? How is it transformed into different products?

Here are the top methods and processes used to transform the pure metallic aluminum into different products.

The Extrusion Process

The extrusion process use aluminum ingots (long aluminum rods). During extrusion, the ingot is first heated to the point that the ingot is formable. It is then pressed through a dies (a shaped tool or mold), causing the heated aluminum to come out the other way by taking the shape of the mold. Think of it like pushing dough through a short pipe. The extrusion process offers unlimited possibilities for forming aluminum into different shapes.

The Rolling Process

The rolling process exploits aluminum’s ductility by compressing ingots into sheets and creating rolled products such as foil, strip, plates, and other products. Thanks to high ductility, aluminum foils can be rolled from 2-6 mm up to 60 cm, with the end result being as thin as 0.006 mm without making it porous.

Recycling and Forming Foundry Alloys

One of the most astonishing qualities of aluminum is its recyclability. It can be recycled countless times with 100 percent efficiency, without losing its property or weight. Additionally, being an exceptional conductor of heat, recycling of scrap aluminum requires just 5 percent of the energy that is needed to smelt new aluminum through the Hall-Heroult process, making it a highly soft after raw material even after becoming waste.

Anything aluminum can be recycled, and the recycled aluminum can be sued to make completely new products. This means, bicycles, computers, boats, automobiles, aircrafts, cans, and appliances, among others can be recycled to create anything for instance, thousands of cans can be recycled to create parts for a helicopter, and so on.

Aluminum metal can be melted and re-melted to form different objects promising same properties as that of the original, pure metal. This is where aluminum foundry alloys come in. They can be easily cast in different shapes, allowing the alloys to be easily re-melted where needed to fit the designs.

The Magic of Aluminum — Some Uses

Aluminum is used across every facet of our lives. It is used in products we use regularly, ranging from window frames, kitchen utensils, can, and foils, to aircrafts, electronic equipment, and super computers.

Then we have the engineered alloys of aluminum. They can significantly increase its strength and reduce its weaknesses (for example, the reason why it can be used in aeroplanes is that the alloys leverage its property of linear expansion while overcoming its weakness of expanding quickly at high temperatures.)

Furthermore, aluminum is used in special equipment such as telescope mirrors (apart from being used in toys and decorative papers). Aluminum forms a highly reflecting coating for both heat and light, preventing deterioration.

Final Words

Lightweight, strong, and non-magnetic, among many other things, aluminum has become the silent hallmark of modern metallurgic achievement. We continue to work its alloys and finding newer uses to leverage its exceptional properties for greater purposes. However, we must remain vigilant and remain abreast of the problems the production of aluminum is posing for our environment, and define a better future for this metal and its responsible use.

The Future of Metallic Yet Non-Magnetic Aluminum

Voluntary Aluminum Industrial Partnership (VAIP) is an organization who has partnered with EPA to find solutions to problems of wastage and pollution that the aluminum industry if facing. IN the U.S, virtually all of the aluminum producers its members, and are committed to the focus of VAIP on researching and developing inert (chemically inactive) electrodes for the reduction process (smelting). New achievements, such as the titanium-diboride-graphite electrodes show promise and shows up to 25% reduction in energy use during the smelting process.

We are committed to working closely with the VAIP and taking up the corporate responsibility of bringing better and greener products for our clients and processes for the industry.